objects that can be modeled using only object mode
Since this question is very unlike Blender.SE I am going to use a tool from other sites. Note, that this answer assumes, that the students have a basic interest and intrinsic motivation.
Frame Challenge
I think there are two dangerous assumptions in your question.

Students should only move on from object mode, when are comfortable using it.
You (the teacher) have to provide the content.

1. Show them a limited, but varying range of tools.
Moving solids around is too tedious to practice, untill you are a master of moving solids around.
Even for the very first lesson, I would include.

Materials: Just a single color, no nodes, no specular.
Edit Mode: Just the basic options in edit mode: GRS. They are the same as in object mode, but allow me to modify my starting blocks. Shearing a cube in object mode is complicated but very simple in edit mode.

Only teach the most basic interaction with an aspect. Students who wish to continue without fancy materials won't waste their time. Students are into complex materials will figure it out on their own or can request a 1 on 1. Students who want more faces in edit mode will be motivated to do their own research.
2. Let the students provide the content.
You, as the teacher, can set the task. It can simply be &quot;modeling&quot;. It can also have a theme, like summer, space, vegetables ...
Anyone, who is willing to invest himself, can provide content. The students themselves will have to figure out, what they are able to model and what they want to include in their scene. Dictating their every move will only create an army of robots. Provide help with the tools but let them create their own content.
There are numerous games/activities to get going on ideas of what you want to create, but that is not in the scope of this answer.
Back to the question.

Anything
Using only solids, you can create anything. Hollow objects seems hard, but who knows what crazy solutions a student could come up with. Give them an example and set some constraints (time, content), but let individuals figure out their individual interests by themselves.
The following scene was create by only duplicating solids. I didn't even use snapping, I just aligned the view carefully. (Setting them up in a scene with an HDRI could be nice though.)